The present exemplary embodiment relates to document processing systems such as printers, copiers, multi-function devices, etc., and more particularly to mitigation of side 1 to side 2 process magnification errors (sometimes referred to as “show-thru” or “see-thru” error) in printing engines and duplex printing methods for printing images on two sides of a printed substrate. This form of magnification error is seen as a difference in the image size for images printed on two different sides of a printed substrate, and is unacceptable in many customer applications. Show-thru errors are mainly caused by the substrate shrinking when fed through a fusing station after the image is transferred to the first side of the substrate and before an image is transferred to the second side. In particular, conventional duplex printing systems include a duplex routing station and media inverter downstream of the fusing station that re-introduce a printed sheet into the transfer path before the once-printed sheet has had time to be re-acclimated to the ambient temperature. The side 2 image is then transferred to the shrunken paper and becomes larger relative to the side 1 image once the paper resumes the original size. This results in a process magnification error evident as a show-thru discrepancy between the image sizes on either side of the substrate, with the side 2 image appearing larger than the side 1 image in the process direction.
The error can be addressed somewhat by shifting the margin for the side 2 image in order to evenly distribute the magnification error equally on both sides. However, this approach does not reduce the process magnification error, but instead makes it less apparent upon visual inspection. Image data may be manipulated to artificially shrink the side 2 image, for instance, by removing certain data, but this leads to image defects.
A technique to implement a side 1 to side 2 image to paper registration system in a digital printer is described in U.S. application Ser. No. 12/177,376, filed Jul. 22, 2008. That approach applies a different image magnification to side 2 that is used to compensate for paper shrinkage due to moisture lost from a fuser and other sources by implementing a method to change raster output scanner (ROS) motor polygon assembly (MPA) speed when imaging side 2 of a sheet compared to side 1. However, to implement the side 1 to side 2 registration system described above an apparatus to adjust ROS MPA rotational velocity in 0.01% increments is necessary.
Consequently, a need remains for improved printing systems and duplex printing techniques by which the adverse effects of side 1 to side 2 process magnification errors can be mitigated.